1. Field of the Invention
The present invention relates to systems and methods for turbocharging and providing exhaust gas recirculation for internal combustion engines.
2. Background Art
Sizing a turbocharger for a particular engine application traditionally requires compromises to achieve a fast transient response from idle with a desired boost when launching a vehicle, especially when using exhaust gas recirculation (EGR), while providing sufficient air handling capacity at high engine speeds. Generally, a fast transient response requires a smaller turbo with lower inertia. However, a smaller turbo has limited air handling capacity for higher engine speeds. A turbo with sufficient airflow capacity for higher engine speeds generally has larger mass and inertia leading to noticeable turbo delay or lag and a less desirable transient response.
EGR handling and control strategies typically require similar compromises. At light load and low engine speed, it is desirable to increase compressor airflow so the compressor operates away from the surge limit to provide more boost. As such, it is desirable to use low pressure EGR supplied to the inlet of the compressor, preferably from downstream of the turbine to maximize turbine speed and compressor airflow. However, at higher engine speeds with higher boost and higher intake manifold pressures, it is desirable to use high pressure EGR supplied from upstream of the turbine to the outlet of the compressor to avoid choking in the compressor.
Various types of emission control devices used with turbocharged engines require a relatively high temperature to operate efficiently. However, the higher airflows and thermal inertia associated with single or twin turbochargers may often require additional warm-up time before the emission control devices reach desired operating temperatures.
A number of solutions have been proposed to address one or more of these problems. For example, single variable geometry turbochargers (VGT) or variable nozzle turbochargers (VNT) have been developed having an adjustable turbine nozzle orifice size or adjustable vane angle to make the turbine more efficient and provide higher boost at lower mass flows. However, to accommodate the higher mass flow capacity at higher engine speeds requires a larger rotor with corresponding inertia resulting in a slower transient response. To improve transient response, twin (primary/secondary) VGT's may be used in a sequential parallel operating mode as described in U.S. Pat. No. 6,055,812, for example. The smaller primary VGT has lower inertia and responds faster at vehicle launch with the larger secondary VGT joining the primary at higher engine speeds and airflows. However, this configuration includes some turbo lag or transient delay for speed-up or spooling of the secondary VGT, which is idled or not used at low engine speeds.
Another approach uses a two-stage turbocharger operated in parallel, such as described in U.S. Pat. Nos. 5,063,744 and 5,142,866, for example. In this configuration, exhaust air is fed into a small primary turbine and then a larger secondary turbine while the ambient air is compressed in a large secondary compressor followed by a second stage smaller compressor. Similar to the parallel twin configuration, the small primary turbocharger has a lower inertia for operation at lower mass flows while the secondary turbocharger has sufficient capacity for higher engine speeds and airflows. A bypass valve is used to shutdown the primary compressor and to connect the output of the secondary compressor to the intake when the air flow reaches a predetermined threshold. To handle airflow at higher engine speeds in this configuration, the required sizing and associated inertia of the secondary turbocharger compromises transient performance of the primary turbocharger leading to undesirable turbo lag or delay.